When GPS (Global Positioning System) technology was initially launched, it was mainly used by the military, as well as the aviation and marine industries. While aircraft navigation systems use GPS for more accurate routing, and the automotive industries use it for fleet management or in-car navigation systems, GPS-enabled hand-held devices have also proven to be highly useful for civilians. Now, GPS technology is on most mobile phones, allowing users to figure out exactly where they are or to get turn-by-turn directions to their desired destinations. [1]
The GPS receiver's primary tasks are measurement of range and range-rate and demodulation of the navigation data, which is the 50/bits data stream modulated onto the GPS signal. The GPS signal format is known as the spread spectrum. [1]
Spread spectrum signals were designed for use in digital communication systems to overcome interference, whether intentional or unintentional. GPS employs binary phase shift keying direct sequence spread spectrum (BPSK DSSS). In DSSS, the information-bearing signal is spread over the entire channel bandwidth in a manner that appears random. DSSS uses a 11-chip long Barker sequence which repeats periodically per symbol. The chip rate is 11 Mchips per second, and the symbol rate is 1 Msymbols per second. The processing gain is fairly slow, particularly in the DSSS mode. [2]
Spread spectrum signaling takes a data signal D(t) of bandwidth B(d) that is modulated on a sinusoidal carrier, and then spreads its bandwidth to a much larger B(s), where B(s) >>B(d). By multiplying the data-modulated carrier by a wide bandwidth-spreading waveform s(t), bandwidth spreading can be obtained. [3]
The following three characteristics outline the fundamentals of a spread-spectrum: (1) the data are modulated onto the carrier in a way that the transmitter signal has a much larger bandwidth than the information rate of the data, (2) the "a priori," or deterministic signal, is utilized by the transmitter to modulate the information signal and spread the spectrum of the transmitted signal, and (3) the receiver draws a parallel between the received signal and the deterministic signal in the process of demodulating the data. In such a method, the receiver recovers the transmitted data. [1]
Most of the error that limits measurement accuracy has to do with the satellite or propagation medium, and thus are out of the control of the receiver. Such errors include satellite clock errors, ephemeris problems, or atmospheric delays. Sensitivity to thermal noise, interference, and multipath is highly dependent upon the receiver architecture, making it highly important to construct the architecture in a way that minimizes risk of error. [1]
© Danielle Rasooly. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. S. Braasch and A. J. van Dierendonck, "GPS Receiver Architectures and Measurements," Proc. IEEE 87, 48 (1999).
[2] "Spread Spectrum Signals for Digital Communication," in Wiley Encyclopedia of Telecommunications, ed. by J. G. Proakis (Wiley-Interscience, 2003).
[3] B. W. Parkinson and J. J. Spilker Global Positioning System: Theory and Applications (Am. Inst. Aeronautics and Astronautics, 1996), pp. 57-65.